Christy Brown once wrote: 'From the gutter of my defeated dreams you pulled me to heights almost your own.'

It was August 1968 and the poem was a dedication to his mother, Bridget, whose recent death had plunged Christy 'into the worst depression he would ever suffer'. He had just lost an extraordinary woman.

This might be the story of Christy's life, but it is also a tribute to Bridget, a woman whose determination, love and dogged refusal to send her son to a home allowed him to flourish as a writer, poet and painter against all the odds. In this powerful biography, Georgina Louise Hambleton paints a wonderful image of a mother's unswerving devotion.

Christy was born in 1932 with cerebral palsy after suffering partial suffocation during birth. It was a crippling disability that meant, though 'his body was almost useless, his mind was perfect'.

In the end, it was his left foot, the only part of his body that he could control, that unlocked Christy's potential. Bridget spent hours helping him learn to read and write in a time when education for the disabled was simply not an option. Two bestselling books later and Christy had proved all the doctors wrong.

Hambleton captures that story, portrayed by Daniel Day-Lewis in the Oscar-winning film My Left Foot, beautifully. But this is also a story of 'depression, isolation and physical suffering', revealing a darkness in later life that the film-makers either did not know about or chose to ignore. They depicted Christy as marrying a nurse and living happily until his death in 1981. Not so, argues Hambleton. Knitting together the memories of his siblings and closest friends, the author has built a portrait of a man who lived his later life in an angry, alcoholic haze, married to a prostitute and lesbian who had affairs and neglected him.

Mary Carr took Christy away from his friends and family and from his beloved Dublin, according to Hambleton. He ended a 10-year affair with a married woman, Beth Moore (depicted here as a beautiful love story) to be with her.

In his final years, those he was once closest to barely saw him. In the end, Christy died at the age of 49 after choking on a dinner of lamb chops. The bruises found on his body even suggest Mary was abusive in the end.

'It seems, though, that the relationship slowly eroded his soul, destroying his art and then him,' says Hambleton. It is certainly a dark and fascinating twist to the tale of Christy Brown, although it is notable that the voices of Mary's friends or family don't feature in the book.

This is a moving portrayal of an astonishing man with a brilliant brain trapped inside a damaged body, who accomplishes more than most able-bodied people could even dream of.